Heck, I'd suggest clicking the link below and downloading it immediately. The Science of Jiu Jitsu has been one of the best Brazilian Jiu Jitsu instructional sets I have ever seen. Below is the 45 minute first volume in the set about Defending the Guard Pass.

He demonstrates each pass, then explains the defense for them- It's really the best video on guard passes and defenses I've encountered, probably. I wish he had a bigger section on half guard passes over the bane of Gi fighters everywhere- the Toreador (...aka the Toreana/Bullfighter) Pass, but regardless of that one fault, I suggest you watch it immediately.